Tiny marine organism lives double life to survive extinction
1 July 2009, by Sara Coelho
Two separate species of tiny marine organisms, a sea-floor dweller and an ocean surface swimmer, are one and the same despite radical differences in lifestyle.
The surprising discovery suggests that the group survived the Cretaceous mass extinction by abandoning the poisonous oceans for a refuge in the relative safety of the sea-floor.
Foraminifers have lived throughout the oceans for many millions of years. They are microscopic, single-celled organisms with tiny shells and are so abundant in marine sediments that they have been used to trace long-term changes in the ocean's environment. Scientists have recognised two main groups: the benthics live in sea-floor sediments, while the planktics inhabit surface ocean waters as part of the plankton.
Dr Kate Darling, from the University of Edinburgh, and colleagues discovered that a planktic species living 600 nautical miles offshore in the Arabian Sea - the Streptochilus globigerus (pictured) - is genetically identical to a benthic species called Boliviana variabilis, found off the coast of Kenya.
The foraminifers survived the Cretaceous extinction in the sediment, not in the plankton.
Dr Kate Darling,
University of Edinburgh
'This was a very surprising discovery,' says Darling. 'It's the same biological species but it is living two separate lives: on the shallow continental shelf and as plankton at the surface of the open ocean.'
The findings were so surprising that Darling and her team decided to investigate further - were the foraminifers caught in the Arabian Sea really at home in the ocean, or were they just driven there by winds and currents from the coast?
Two for one
The answer lies in the chemistry of the foraminifer's chambered shells, which are made up of calcium carbonate but can also take up a small amount of magnesium. The proportion of magnesium and calcium found in each shell depends on water temperature and can be used as a virtual thermometer - more magnesium means that the foraminifer grew in warmer water.
Foraminifer shells are divided up into compartments, with new chambers added as the organism grows. By looking at the magnesium and calcium proportion, Darling and her team discovered that there is a difference of 3°C in water temperature between juvenile and adult chambers. The findings suggest that the young foraminifers most likely grew in the hot ocean surface and migrated as adults to cooler and deeper waters.
This means that an alleged benthic species can live and grow in an open ocean planktic environment. 'They can live equally well in both places,' says Darling. The ability to occupy radically different habitats is known from diatoms, another group of small aquatic organisms. But this is the first time it was recorded for foraminifers.
Survival lesson number one: stay out of the plankton
Scientists think that all modern planktic foraminifers descend from a bottom-dwelling species that switched to a planktonic lifestyle sometime during the Mid to Early Jurassic, between 160 and 200 million years ago.
After the jump, the group diversified and new planktic species quickly colonised the oceans. Planktic foraminifers became very abundant, but only a small part survived the late Cretaceous extinction that killed off the dinosaurs. All modern planktic foraminifers are believed to descend from these lucky few.
Darling's findings complicate the story considerably. If some species can switch between free-swimming and bottom-dwelling lifestyles, then it's possible that 'most planktic foraminifers may have survived the KT [late Cretaceous] extinction in the sediment, not in the plankton,' says Darling.
It seems likely that the foraminifer species which had the ability to occupy both habitats survived on the sea-floor, avoiding the meteor impact catastrophe in the oceans above, argue the authors in the report published this week in the Proceedings of the National Academy of Sciences. When the oceans returned to normal, the survivors were able to recolonise the ocean surface once more.
Darling is still working on this idea, but it's very telling that 'two of the three types that survived the KT extinction belong to the same kind of foraminifera' she says, referring to the two species now revealed as one.
Kate F. Darling, Ellen Thomas, Simone A. Kasemann, Heidi Seears, Christopher W. Smart, and Christopher M. Wade. Surviving mass extinction by bridging the benthic/planktic divide. PNAS published online ahead of print 29 June 2009.
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